feat: meta-model visualization and editing in viewer

[rajnavakoti]

Description

The typed meta-model (18 entity types, 6 knowledge layers, 55 relationship types) is CB's structural backbone, but it's invisible to users. It lives in code (meta_model.py) and a YAML config (meta-model.yaml) that most users will never open. Per context block, users should be able to see and edit their domain's ontology visually.

Acceptance Criteria

• Viewer has a dedicated meta-model page (or section) showing all entity types, layers, and relationship types
• Visual representation of the type hierarchy: layers → types → allowed relationships
• Per-context-block view: which types are populated, how many entities per type, coverage per layer
• Editable from UI: users can add custom entity types, adjust layer assignments, define new relationship types
• Changes persist back to meta-model.yaml (or a per-block override config)
• Entity type cards show: count, average confidence, example entities, allowed relationships

Out of Scope

• Formal OWL/RDF reasoning or inference engine
• Changes to the extraction pipeline's meta_model.py (that stays as source of truth for extraction; this is the viewer/curation surface)